The present invention relates to electronic circuits and, more particularly, to differential amplifiers. A major objective of the present invention is to provide a differential amplifier system that can respond rapidly to changes of differential input signals to accommodate high information transfer rates.
Societal operation and progress is propelled by the propagation of information. Information can be transferred through a variety of media, including air, electrical conductors, optical fibers, etc. Typically, demands for access to communication pathways soon exceed capacity. Thus, there is a strong incentive to maximize the rate at which information can be transmitted along a communication pathway.
The maximum information rate is generally constrained by the highest frequency that can be handled by a communication system. Information transmitted at a rate above this highest frequency will be lost. Therefore, information transmission is limited to rates low enough to substantially avoid such information loss. Providing for higher communication rates then requires a system with a greater maximum operating frequency.
One of the most pervasive approaches to communications is the transmission of information encoded as a time-varying voltage along an electrical conductor. Electrical communication systems are subject to information loss due to voltage signal attenuation and distortion. Attenuation is a result, at least in part, of electrical impedances associated with the conductor. These impedance include termination resistances used to avoid signal reflections due to impedance mismatches and the distribued capacitance between the conductor and ground. Distortion can be the result of ambient electric fields that bias the voltage waveform being transmitted.
Distortion can be addressed to a significant extent by using balanced-line communication systems. A balanced-line communication includes a transmitter with complementary outputs, a pair of transmission lines to convey respective ones of the complementary outputs, and a receiver. The receiver typically includes a differential amplifier to convert attenuated complementary waveforms into a regenerated waveform or a regenerated pair of complementary waveforms. The output of the differential amplifier reflects the difference in the complementary inputs. Any biases applied in common to both of the balanced lines is thereby cancelled. Such balanced line systems are often employed, for example, to communicate digital data in local area networks.
The response of the differential amplifier to changes in its inputs is not instantaneous. The response time depends on the magnitude of the differential driving the receiver, the speed of a differential amplifier's active elements as well as the size of the load to be driven by the amplifier. If the inputs change a second time before the output has properly represented the first change, information is lost. Thus, the communication rate must be maintained at or below the inverse of the transition time so that each signal transition can be responded to properly by the receiver.
Higher communication rates are potentially attainable using greater driving voltage differentials, but only at the risk of impedance mismatches. The impedance mismatches causes signal reflections that can interfere with the intended transmission. Higher communication rates can be accommodated by using faster active elements and larger bias currents, which provide for higher output slew-rates and thus faster transitions. However, there is still a demand for higher communication rates at any given level of component speed and power availability.
What is needed is an economical high-performance receiver for balanced-line communication systems. In other words, an approach is required that improves slew rate for a given driving voltage differential, given active element speeds, and given power specifications so as to provide for higher frequency operation and greater information communication rates.